Conventionally, blood pressure measurement by the volume compensation method has been developed as a method capable of non-invasively and simply measuring blood pressure. Under the volume compensation method, an artery is compressed by a cuff from the outside of a living body, so that a volume of the artery pulsating in synchronization with a heartbeat is always maintained to be constant. By maintaining the volume of the artery to be constant, pressure (cuff pressure) for compressing a measuring site and internal pressure of the artery of the measuring site, that is, blood pressure, are equilibrated. By detecting the cuff pressure when this equilibrium state is maintained, a blood pressure value is continuously obtained.
Therefore, in the volume compensation method, two points of detection of a volume value when the artery is in an unloaded state, that is, a control target value (hereinafter, also referred to as “V0”), and maintenance of this unloaded state (servo-control) are important. Particularly, decision of V0 is highly important as V0 directly influences precision of blood pressure measurement.
As a decision method of V0, for example, as disclosed in Japanese Unexamined Patent Publication No. 1984-156325 (patent document 1), there is a method of deciding V0 by gradually compressing the artery by a cuff and detecting a maximum point of an artery volume change signal obtained at that time.
Conventionally, there is also a proposal for realizing shortening of a time required for the decision of V0. For example, in Japanese Unexamined Patent Publication No. 2008-36004 (patent document 2), high-frequency small pressure oscillation (such as sine wave pressure oscillation of 20 Hz, 10 mmHg) is superimposed on the cuff pressure, and V0 is decided by detecting a maximum point of an artery volume change due to the high-frequency small pressure oscillation.
It should be noted that with regard to shortening of a time for the blood pressure measurement, the following technology is disclosed in U.S. Pat. No. 6,802,816 (patent document 3) although different from the volume compensation method. That is, the cuff pressure is rapidly increased and reduced within one heartbeat, so that a feature point on a pulse wave indicating an instant when the artery internal pressure is equal to the cuff pressure is detected. The cuff pressure at a time point when the feature point is detected is decided as the blood pressure value.    Patent Document 1: Japanese Unexamined Patent Publication No. 1984-156325    Patent Document 2: Japanese Unexamined Patent Publication No. 2008-36004    Patent Document 3: U.S. Pat. No. 6,802,816